With the advances of portable electronic devices, demands for light and high capacity batteries have increased, and as secondary batteries capable of satisfying such requirements, development of lithium-sulfur batteries using sulfur series materials as a cathode active material has been actively progressed.
Lithium-sulfur batteries are a secondary battery using sulfur series compounds having sulfur-sulfur bonds as a cathode active material, and using alkali metals such as lithium, or carbon-based materials capable of intercalation and deintercalation of metal ions such as lithium ions as an anode active material, and store and generate electric energy using an oxidation-reduction reaction reducing an oxidation number of sulfur (S) as sulfur-sulfur (S—S) bonds are broken during a reduction reaction (discharge) and forming sulfur-sulfur (S—S) bonds again as an oxidation number of sulfur (S) increases during an oxidation reaction (charge).
Specifically, an oxidation-reduction reaction of lithium and sulfur in a lithium sulfur battery may be expressed as the following reaction formulae.2Li+S8(solid)Li2S8(solution)2Li+Li2S8(solution)2Li2S4(solution)2Li+Li2S4(solution)2Li2S2(solution)2Li+Li2S2(solution)2Li2S(solid precipitate)
When referring to the reaction formulae, it is seen that lithium polysulfide, a new reaction product, is produced during an oxidation-reduction reaction of lithium and sulfur. Reaction capacity of sulfur capable of being used in an actual lithium sulfur battery is very low of approximately 840 mAh/g, an approximately half of theoretical capacity, due to irreversible reaction properties of some polysulfide. As a result, a lithium sulfur battery using sulfur as a cathode active material has a problem of low battery capacity.
In addition, lithium metal as an anode has an advantage of being light and having excellent energy density, but has a problem in that a cycle life characteristic is reduced due to high reactivity. In view of such a problem, researches on the formation of a protective layer capable of protecting a lithium metal surface have been recently progressed. As such a protective layer, inorganic protective layers and polymer protective layers are included, and among these, lithium phosphorus oxy-nitride (LiPON), a lithium ion conductor, has been studied representatively. However, the LiPON protective layer is formed using a sputtering method under nitrogen gas atmosphere, and when intending to form directly on a lithium metal surface, nitrogen gas and lithium metal react leading to a problem of faulting a black porous lithium complex compound having very poor binding strength on the lithium metal surface as a byproduct. In addition, when forming a polymer protective layer, a reaction sometimes occurs between a used organic solvent used when forming the protective layer and lithium metal.
Accordingly, development of materials for increasing capacity by increasing an electrochemical oxidation-reduction reaction has been required in a lithium sulfur battery.